3D Structural Analysis of Otu Field, Niger Delta, Nigeria (PDF ...

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Physical Science International Journal 7(2): 114-126, 2015, Article no.PSIJ.2015.063

ISSN: 2348-0130

SCIENCEDOMAIN international www.sciencedomain.org

3D Structural Analysis of Otu Field, Niger Delta, Nigeria

T. N. Obiekezie1* and E. E. Bassey1

1Department of Physics and Industrial Physics, Nnamdi Azikiwe University, Awka, Nigeria.

Authors’ contributions

This work was carried out in collaboration between both authors. Author TNO designed the study, wrote the protocol, and wrote the first draft of the manuscript. Author EEB managed the literature

searches and carried out the analysis. Both authors read and approved the final manuscript.

Article Information

DOI: 10.9734/PSIJ/2015/16512 Editor(s):

(1) Mohd Rafatullah, Division of Environmental Technology, School of Industrial Technology, Universiti Sains Malaysia, Malaysia.

(2) Christian Brosseau, Distinguished Professor, Department of Physics, Université de Bretagne Occidentale, France. Reviewers:

(1) Valiya M Hamza, National Observatory, Rio de Janeiro, Brazil. (2) Mohamed I. Abdel-Fattah, Geology Department, Faculty of Science, Suez Canal University, Egypt.

(3) Peter A. Alao, Department of Applied Geology, Ruhr University Bochum, Germany. Complete Peer review History: http://www.sciencedomain.org/review-history.php?iid=1054&id=33&aid=9658

Received 5th February 2015 Accepted 12

th May 2015

Published 8th

June 2015

ABSTRACT

3D structural analysis was carried out to evaluate the subsurface structures and hydrocarbon trapping potential of Otu Field, Niger Delta using 3D seismic and well log data. Lithologies and hydrocarbons were initially delineated with the aid of gamma ray, deep resistivity, neutron and density logs of wells. The spatial and depth distribution of lithologies were correlated across the wells in the study area. Network of faults were interpreted allowing identification of growth faults which are listric in character. Three horizons, C10, D10 and D31 were identified and mapped to produce the structure maps. The structure maps of the tops of the reservoirs revealed that the hydrocarbon structures are fault assisted anticlinal structures and they correspond to the crest of the rollover anticlines on the seismic sections. The RMS amplitude attribute extracted on the surfaces revealed bright spots on the region of the anticlinal structures which indicates that the field has economic explorable hydrocarbons accumulations.

Keywords: Seismic; horizons; listric; structures; reservoir; Niger delta.

Case Study

Obiekezie and Bassey; PSIJ, 7(2): 114-126, 2015; Article no.PSIJ.2015.063

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1. INTRODUCTION The Niger Delta is ranked among the major prolific deltaic hydrocarbon provinces in the World and is the most significant in the West African continental margin [1]. Several workers have carried out structural interpretations of different fields of the Niger Delta using seismic and well log data [2-5]. This is as a result of the high demand for hydrocarbon products since the 20

th century.

The goal of oil and gas exploration is to identify and delineate structural and stratigraphic traps suitable for economically exploitable accumulations. Hydrocarbons reservoirs are found in geologic traps, that is, any combination of rock structure that will keep oil and gas from escaping either vertically or laterally [6]. These traps can either be structural, stratigraphic or a combination of both. Structural traps can serve to prevent both vertical and lateral migration of the connate fluid [7]. Examples of these include rollover anticlines and flanks of salt domes [8]. Stratigraphic traps include sand channels, pinch outs, unconformities and other truncations [9]

According to [10], majority of the traps in the Niger Delta are structural and to locate them, horizons are picked and faults mapped on the seismic in-lines and cross-lines to produce the time structure maps. This can reveal the structures that can serve as traps for the hydrocarbons [8]. In this study, 3D seismic data were integrated with well logs to delineate the geologic structures and hydrocarbon trapping potential of the study area. In addition, amplitude attributes analysis that indicates bright spot which is a direct hydrocarbon indicator (DHI) was carried out. The bright spot is a valuable mapping tool because it allows visual identification of features related to the presence of hydrocarbons directly on seismic traces.

2. LOCATION AND GEOLOGY OF THE

STUDY AREA Otu field is an onshore field located in the Western part of the Niger Delta, Nigeria and lies between latitudes 5°N and 6°N and longitudes 5°E and 6°E (Fig. 1). The field covers approximately 720 km

2.

The Niger Delta is ranked among the major prolific deltaic hydrocarbon provinces in the world and is the most significant in the West Africa continental margin. The Niger delta basin is situated on the continental margin of the Gulf of Guinea between latitude 4°-9°N and longitude 4°-9°E. It is composed of overall regressive clastic sequence, which reaches a maximum thickness of about 12000 m [11]. The sedimentary sequences in the subsurface of the Niger Delta have been modified by numerous transgressions which occurred from time to time breaking the continuity of the main overall regression [12].

The Niger Delta consists of three broad formations (Fig. 2) representing prograding depositional facies that are distinguished mostly on the basis of sand-shale ratios [13]. These are: the basal Paleocene to Recent pro-delta facies of the Akata Formation, the Eocene to Recent, paralic facies of the Agbada Formation, and the Oligocene to Recent, fluvial facies of the Benin Formation [14]. Petroleum in the Niger Delta is produced from the sandstone and unconsolidated sands predominantly in the Agbada Formation [13]. Characteristics of the reservoirs in the Agbada Formation are controlled by depositional environment and by depth of burial. Known reservoir rocks are Eocene to Pliocene in age, and are often stacked, ranging in thickness from less than 15 m to 10% having greater than 45 m thickness [11]. The thicker reservoirs likely represent composite bodies of stacked channels [10].

Based on reservoir geometry and quality, Kulke [15] describes the most important reservoir types as point bars of distributary channels and coastal barrier bars intermittently cut by sand-filled channels. [16] describe the primary Niger Delta reservoirs as Miocene paralic sandstones with 40% porosity, 2 darcys permeability, and a thickness of 100 m. The lateral variation in reservoir thickness is strongly controlled by growth faults; the reservoir thickness towards the fault within the down-thrown block [17]. The grain size of the reservoir sandstone is highly variable with fluvial sandstones tending to be coarser than their delta front counterparts; point bars fine upward and the barrier bars tend to have best grain sorting. Most of this sandstone is nearly unconsolidated, some with minor component of argillo-silicic cement [15].


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Fig. 1. Map of Niger Delta showing the location of the study area (Otu Field)

Fig. 2. Stratigraphic column showing the three formations of the Niger Delta [10]

Most known traps in Niger Delta fields are structural although stratigraphic traps are not uncommon (Fig. 3). The structural traps developed during synsedimentary deformation of the Agbada paralic sequence [11]. Structural complexity increases from the North (earlier formed depobelts) to the South (later formed depobelts) in response to increasing instability of the under-compacted, over-pressured shale.

Doust and Omatsola [10] described a variety of structural trapping elements, including those associated with simple rollover structures; clay filled channels, structure with multiple growth faults, structures with antithetic faults, and collapsed crest structures (Figs. 3 & 4). Stratigraphic traps occur on the flanks of the delta [13]. Pockets of sandstone occur between diapiric structures in the region.


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The primary seal rock in the Niger Delta is the interbedded shale within the Agbada Formation. The shale provides three types of seals – clay smears along faults, interbedded sealing units against which reservoir sands are juxtaposed due to faulting, and vertical seals [10]. On the flanks of the delta, major erosional events of early to middle Miocene age formed canyons that are now clay-filled (Fig. 2). These clays form the top seals for some important offshore fields [10].

3. METHODOLOGY

The data available for this study include 3D seismic volume in SEGY format, a composite well logs comprising of gamma ray (GR), resistivity deep (RESD), Sonic (BHC), density (FDC) and neutron (NEU) logs, and checkshots data. Petrel software was used to interpret the seismic data and to generate maps as well as well log cross sections.

The gamma ray log (GR) was used to identify the lithology (sand and shale) because it is believed that in the Niger Delta, hydrocarbon reservoirs are found within sand units. The tops of the formation were correlated across the wells in the field and base of each formation was created to define vertical extent of the formation. Hydrocarbons were delineated on the formations with the aid of deep resistivity log. Synthetic seismogram was generated by convolving the

reflectivity derived from sonic and density logs with the wavelet derived from seismic data (Fig. 5). The sonic log was calibrated (corrected) with checkshots before combining with the density log to produce reflection coefficient (Fig. 6)

The synthetic seismogram was used for tying the well data and seismic data. This tie formed the first step in picking events, which corresponded to the tops of the sands for interpretation. Picking of faults, mapping of horizons and loop tying were carried out manually.

Faults were recognized from the seismic section by distinct discontinuity or abrupt jump of seismic reflection events. The interpreted faults were quality checked on the variance time slice and corrected/assigned. Slices were moved up and down in time to confirm fault consistency. The variance attribute is an edge imaging detection method. By using the synthetic seismogram created previously, the tops of the sands identified on the logs were tied to the seismic reflection events on the seismic sections. Three horizons were interpreted based on the tops and were traced through the whole seismic volume. The horizons were interpreted on every 10 in-lines and 10 cross-lines and seismic seed grids were generated. The grids were infilled by interpolation.

Fig. 3. Examples of Niger Delta oil field structures and associated trap types (Redrawn from [10])

Fig. 4. Conventional trapping configuration in the Niger Delta (Modified from [17])


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Fig. 5. Wavelet extraction for Otu36

Fig. 6. Sonic calibration Otu36

The RMS amplitude attribute was extracted for each horizon. Time structure maps were produced using the interpolated seismic seed grids for each horizon. The time maps were then converted to depth maps using a simple velocity model.

4. DISCUSSION OF RESULTS Well logs study revealed a few number of sand reservoirs of which three C10, D10, and D31 were mapped at depth of 4512 ft, 5337 ft and

5536 ft respectively. The gross thickness of the C10 reservoir sandstone formation ranges from 45 ft to 78.5 ft. Since the reservoir was intercalated with shale, the net thickness varied between 11.5 ft and 54.5 ft. The gross thickness of the D10 reservoir varied between 55.5 ft and 103 ft; while the net thickness varied between 13 ft and 51 ft. The gross thickness of D31 reservoir varied between 127.5 ft and 273 ft and the net thickness varied between 11 ft and 114 ft. A log correlation connecting all the wells across the area is shown in Fig. 7.

The synthetic seismogram generated revealed that Otu wells have a good time depth tie with a trough to trough and peak to peak match. Wellto-seismic tie revealed that the mapped hydrocarbon bearing reservoirs lie on the trough of the rollover anticlines on seismic sections. Fig. 8 shows the synthetic seismogram of Otu36 and the mapped well tops. Several faults were identified and marked with different colours. This revealed three major growth faults which are listric in nature and concave basinfaults mapped are synthetic and antithetic faults. Displacement of seismic facies across faults increases with depth in the seismic record. The three horizons mapped are characterized by low to high or variable amplitude reflections with moderate to good continuity. There are truncations in some places which are caused by faults. Fig. 9 shows the interpreted faults and horizons on the seismic sections. Fig. 10 shows the variance time slice with faults sticks. From the faults and the horizons interpretedstructure maps were produced. The time structure maps were converted to depth structure maps using the velocity model. The contouring was actually done by joining points of equal depth going round the data with contour interval of 50ft for each surface. Points of equal depth

Fig. 7a

Obiekezie and Bassey; PSIJ, 7(2): 114-126, 2015; Article no.PSIJ

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The synthetic seismogram generated revealed that Otu wells have a good time depth tie with a trough to trough and peak to peak match. Well-

seismic tie revealed that the mapped hydrocarbon bearing reservoirs lie on the trough

n seismic sections. Fig. 8 shows the synthetic seismogram of Otu36 and the mapped well tops. Several faults were identified and marked with different colours. This revealed three major growth faults which are listric in nature and concave basin-wards. Other faults mapped are synthetic and antithetic faults. Displacement of seismic facies across faults increases with depth in the seismic record. The three horizons mapped are characterized by low to high or variable amplitude reflections with

d continuity. There are truncations in some places which are caused by faults. Fig. 9 shows the interpreted faults and horizons on the seismic sections. Fig. 10 shows

faults sticks.

From the faults and the horizons interpreted, time structure maps were produced. The time structure maps were converted to depth structure maps using the velocity model. The contouring was actually done by joining points of equal depth going round the data with contour interval

ace. Points of equal depth

are identified by having the same colour and the depth of each colour is shown in the colour bar in Figs.11,12 and 13. Depth structural map of horizon C10 is shown in Fig.11. The contoured map has values ranging from 3400ft to 61Structural highs are observed at Northand the central part of the field. This area forms a good trapping system thereby increasing retentive capacity for hydrocarbon. The hydrocarbon trapping system in the central part of the field where the wells are located is a faulted rollover anticlines. The low faults throw in the area is responsible for excellent retentive capacity of hydrocarbons. Structural lows are seen in the south-western region and the area is marked with no prospect. Fig. 12 is structural map for horizon D10. The contoured interval value ranges from 4250 Structural highs were observed in the NorthWestern part and the central part serve as good traps for the hydrocarbon accumulation. The hydrocarbon trapping system is still faulted rollover anticlines. In the SouthSouth-Eastern region of the field, structural lows are observed. The depth structure map of D31 horizon is presented in Fig. 13. The D31 horizon is similar in characteristics to the horizon D10 but is located at a considerable deeper depth. They have the same structural style.

Fig. 7a. Well correlation panel of Otu field

; Article no.PSIJ.2015.063

are identified by having the same colour and the depth of each colour is shown in the colour bar in Figs.11,12 and 13. Depth structural map of horizon C10 is shown in Fig.11. The contoured map has values ranging from 3400ft to 6100ft. Structural highs are observed at North-western and the central part of the field. This area forms a good trapping system thereby increasing retentive capacity for hydrocarbon. The hydrocarbon trapping system in the central part

wells are located is a faulted rollover anticlines. The low faults throw in the area is responsible for excellent retentive capacity of hydrocarbons. Structural lows are

western region and the area is 12 is the depth

structural map for horizon D10. The contoured ft to 7200 ft.

Structural highs were observed in the North-Western part and the central part serve as good traps for the hydrocarbon accumulation. The

ing system is still faulted rollover anticlines. In the South-Western and

Eastern region of the field, structural lows are observed. The depth structure map of D31

13. The D31 horizon horizon D10 but

is located at a considerable deeper depth. They

Fig. 7b. Well corre

Fig. 8. Synthetic seismogram of Fig. 14 to 16 shows the RMS amplitude map of the interpreted horizons. The amplitude map was used to know the distribution of high and low amplitude across each horizon and try to find any special features in the study area, such as


120

Well correlation panel of Otu Field contd

Synthetic seismogram of Otu36 and the mapped well tops

Fig. 14 to 16 shows the RMS amplitude map of the interpreted horizons. The amplitude map was used to know the distribution of high and low

orizon and try to find any special features in the study area, such as

lithology and fluid content. The high amplitude zones (red, yellow and green colour) at the Epart of the map indicate the presence of hydrocarbon and correspond to the structural high of the map. The amplitude map for D31


lithology and fluid content. The high amplitude zones (red, yellow and green colour) at the E-W part of the map indicate the presence of hydrocarbon and correspond to the structural

h of the map. The amplitude map for D31

sand didn’t fully correspond to the lithology and this could be due to the search window used, or poor quality data at the deeper zone of the field. A greater part of the central part shows bright

Fig. 9. Seismic Inline showing fault sticks

Fig. 10.


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sand didn’t fully correspond to the lithology and this could be due to the search window used, or poor quality data at the deeper zone of the field. A greater part of the central part shows bright

spot. Bright spots are seen as an indication of hydrocarbon presence [18], the observed bright spots correspond to the rollover structure of the field.

Seismic Inline showing fault sticks and horizons interpreted

. Variance time slice with fault sticks


are seen as an indication of hydrocarbon presence [18], the observed bright

correspond to the rollover structure of the


122

Fig. 11. Depth structure map of horizon C10

Fig. 12. Depth structure map of horizon D10


123

Fig. 13. Depth structure map of horizon D31

Fig. 14. RMS amplitude for horizon C10


124

Fig. 15. RMS amplitude for horizon D10

Fig. 16. RMS amplitude for horizon D31


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5. CONCLUSION The 3D structural analysis of the Otu Field have provided a better understanding of the structural styles and hydrocarbon trapping systems of the field. From the well logs analysis three hydrocarbon bearing reservoirs (C10, D10 and D31) were delineated. The net thickness of the reservoir varies between 45 ft and 273 ft. A network of faults and three horizons were interpreted to generate the structure maps. The main faults in the field are growth faults which are listric in nature. The structure maps, indicates that hydrocarbon accumulations in the field are associated with the structural highs and closures that are faults dependent. These structures correspond to the crest of rollover structure in the field. The amplitude maps revealed bright spots on these regions thereby suggesting economic explorable hydrocarbon accumulations.

ACKNOWLEDGEMENTS The authors are sincerely greatful to the managements of Shell Petroleum Development Company (SPDC) Nigeria, the Department of Petroleum Resources (DPR) Nigeria and Integrated Data Services Limited (IDSL) Nigeria for the various roles played.

COMPETING INTERESTS Authors have declared that no competing interests exist.

REFERENCES 1. Aizebeokhai AP, Olayinka I. Structural and

stratigraphic mapping of Emi field, offshore Niger Delta. Journal of Geology and Mining Research. 2011;3(2):25-38.

2. Opara AI, Anyiam UO, Nduka AV. 3-D Seismic interpretation and structural analysis of Ossu oil field, Northern Depobelt, Onshore Niger Delta, Nigeria. The Pacific Journal of Science and Technology. 2011;12(1):502-509.

3. Adewoye O, Amigun JO, Okwoli E, Cyril AG. Petrophysical and structural analysis of Maiti field, Niger Delta, using well logs and 3D seismic data. Petroleum & Coal. 2013;55(4):302-310.

4. Ihianle OE, Alile OM, Azi SO, Airen JO. Osuoji OU. Three dimensional seismic/well

logs and structural interpretation over ‘X-Y’ field in the Niger Delta Area of Nigeria. Science and Technology. 2013;3(2):47-54.

5. Rotimi OJ, Ameloko AA, Adeoye OT. Application of 3-D structural interpretation and seismic attribute analysis to hydrocarbon prospecting over X- field, Niger- Delta. International Journal of Basic and Applied Sciences. 2010;10(04)28-40.

6. Wan Qin. Reservoir delineation using 3-D seismic data of the Ping Hu field, East China, Thesis, University of Colorado. 1995;6-8.

7. Coffen JA. Interpreting seismic data. Penwell Publishing Company, Tulsa Oklahoma. 1984;39-118.

8. Adeoye TO, Enikanselu PA. Hydrocarbon reservoir mapping and volumetric analysis using seismic and borehole data over “Extreme” field, Southwestern Niger Delta. Ozean Journal of Applied Sciences. 2009; 2(4):429-441.

9. Folami TO, Ayuk MA, Adesida A. Identification of hydrocarbon reservoirs using seismic attributes and geocellular modelling: A case study from “Tyke” field, Niger Delta. Nigeria Association of Petroleum Geologists Bulletin. 2008;30-32.

10. Doust H, Omatsola E. “Niger Delta”. In: Edwards JD, Santogrossi PA. (editors). Divergent/Passive Margin Basins, AAPG Memoir. American Association of Petroleum Geologists: Tulsa, OK. 1990; 48:239-248.

11. Evamy BD, Haremboure J, Kamerling P, Knaap WA, Molloy FA, Rowlands PH. Hydrocarbon habitat of Tertiary Niger Delta. American Association of Petroleum Geologists Bulletin. 1978;62:277-298.

12. Hospers J. Gravity field and structure of the Niger Delta, West Africa. Geological Society of American Bulletin. 2005;76:407-422.

13. Tuttle MLW, Charpentier RR, Brownfield ME. The Niger Delta basin petroleum: Niger Delta Province, Nigeria, Cameroon, and Equatorial Guinea, Africa. Open-File Report 99-50-H. United States Geological Survey, Washington D.C.44; 1999.

14. Short S, Stauble G. Outline of geology of Niger Delta. American Association of Petroleum Geologists Bulletin. 1967;761-768.


126

15. Kulke H. Nigeria, in, Kulke H, ed., Regional petroleum geology of the World. Part II: Africa, America, Australia and Antarctica. Berlin, Gebrüder Borntraeger. 1995;143-172.

16. Edwards JD, Santogrossi PA. Summary and conclusions, in, Edwards JD, Santogrossi PA, eds., Divergent/passive Margin Basins, AAPG Memoir 48: Tulsa, American Association of Petroleum Geologists. 1990;239-248.

17. Weber KJ, Daukoru EM, Petroleum geology of the Niger Delta: Proceedings of the 9

th World Petroleum

Congress, Geology: London, Applied Science Publishers Ltd. 1975;2:210-221.

18. Obiekezie TN. Hydrocarbon exploration in Odo field in the Niger Delta basin Nigeria, using a three- dimensional seismic reflection survey. Scientific Research and Essays. 2014;9(17):778-784.

_________________________________________________________________________________ © 2015 Obiekezie and Bassey; This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Peer-review history: The peer review history for this paper can be accessed here:

http://www.sciencedomain.org/review-history.php?iid=1054&id=33&aid=9658

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